The present invention relates to vibratory conveyors, and particularly to excited base vibratory conveyors which include a trough which is spring mounted to a base wherein the base is excited by a vibration driver, and the trough is vibrated through the spring mounting. Particularly, the invention relates to the balancing and orienting of the forces produced by a linear vibratory driver which act directly on a base, with respect to a center of gravity of a trough which is supported by springs from the base, and with respect to the spring forces which support the trough from the base, to minimize movement of the base, while minimizing the required weight of the base.
Vibratory conveyors are often used by industry to transport bulk materials or small parts, and perform processing operations such as screening, transferring, singulating, heating, cooling, etc., on bulk materials such as food products, chemicals, sand, gravel, foundry parts, and other similar applications. Typical food products conveyed include snack foods such as potato chips, pretzels, nachos, corn puffs, etc., vegetables, cereals and the like.
Vibratory conveyors offer many advantages over alternate conveying means in applications where, for example, sanitary considerations, precise or controlled feed rates, particular process operations, or low maintenance, are important to the end user. Although vibratory conveyor systems can incur a higher initial cost than some available alternate conveying means, vibratory conveyor systems offer advantages which offset the higher cost for many applications. For some applications practical alternative conveying means are not available. It would be advantageous, however, to reduce the cost of vibratory conveyors by reducing manufacturing cost while maintaining effective operation of the conveyor.
There are many design variations of vibratory conveyors, including both single mass and two-mass designs. Variations of the two-mass designs, such as base excited vibratory conveyors, include: crank-driven balanced isolated base designs, such as FMC Corporation""s MHE Operation""s BL and LBL conveyors; and crank-driven counter-balanced base designs.
Certain prior art two-mass conveyor designs are configured such that the directed force exciting the conveyor falls on a line aligning the center of gravities of both the trough and base members of the conveyor, and perpendicular to a face of the drive springs. This design is disclosed in U.S. Pat. No. 4,313,535 for example. This patent discloses an excited base conveyor having a linear forcing function drive means which generates a force drive line that is colinear with a line passing substantially through the center of gravity of the trough mass, conveyor system mass, and the base mass.
As a result of this design approach, it is often necessary, particularly with short conveyors, to add large amounts of weight to the base member such that the plural center of gravities may be brought into alignment. In crank-driven or magnetically excited conveyor designs, the drive force is applied simultaneously to both the base and trough members, and the relationship of the stroke on the base to the design trough stroke is inversely proportional to the weight ratio between the base and the trough. In order to minimize the forces transmitted to the support structure for the conveyor, it is required that the stroke on the base must be kept as small as practical, and so the base structure tends to become quite heavy. Ratios of base to trough weights in these prior designs typically range from 3:1 to 8:1 and more.
In the case of the base excited conveyor, however, the stroke on the base is more dependent on the tuning of the conveyor, and the relationship of the natural frequency of the mass/spring system to the operating frequency of the conveyor. The resultant base weight to trough weight ratio can be in a range of 1.5:1 to 3:1 or more depending on the available force, and thus how close to resonance it is necessary to tune the conveyor to get the desired trough stroke.
Although the excited base conveyor design has an advantage over the crank-driven conveyor from a weight-of-base perspective, it is still possible that a relatively heavy base structure is required for the excited base design. It would be advantageous to provide a design whereby the overall weight of the conveyor, and particularly the weight of the base is minimized.
The vibratory conveying device embodying the present invention includes a trough for transporting materials, a base underlying the trough, the trough supported from the base by a plurality of elongate leaf springs. A linear vibratory drive is connected to the base. The linear vibratory drive generates a linear force along a first line which passes through a center of gravity of the trough. The springs are arranged parallel to a second line which defines a spring angle, with the first line being perpendicular to the second line. The force generated by the linear vibratory driver is equal to and opposite to the reaction force of the plurality of springs. The center of gravity of the base is located at a distance from the first line, i.e., the center of gravity of the base need not be located along the line of linear force which includes the center of gravity of the trough.
According to the invention, for a base excited conveyor design, the dynamic motion of the conveyor can be balanced without having to align the center of gravities of the trough and base to be colinear with the drive line. It is only necessary to balance the spring force of the two-mass system with the force generated by the linear drive, such that the stroke on the base member is virtually zero. Since the base has zero motion, the spring system can be somewhat evenly balanced about the center of gravity of the trough, and no unbalanced moments exist that cause a rotation, or a pitching of the base. Without motion of the base, the support structure has virtually only the static load of the conveyor resting thereon.
By balancing the forces acting on the base, the design of the base is dictated by required structural integrity considerations, and not weight for balancing the stroke of the trough. The base must be strong enough to withstand the imposed loads and have sufficient stiffness to avoid any natural bending frequency of the base structure that would be susceptible to excitation by the operating frequency of the conveyor. Thus, the weight of the base can be made lighter than prior art excited base designs. The base being lighter, the overall weight of the conveyor can be made lighter, representing a lower installation cost for the user and reduced size and complexity of the support structure for the conveyor.
Other features and advantages of the present invention will become readily apparent from the following detailed description of the accompanying drawings, and the appended claims.